The field of the invention is that of transmitting signals by a frequency channel.
It will be recalled that transmission of a signal by a frequency channel more and more often calls upon digital modulation, the main advantage of which is that it permits the use of signal processing algorithms. The purpose of these algorithms is to increase the strength of the signal to be transmitted relative to the propagation channel.
More precisely, the invention relates to a radiofrequency transmitter of the type supplied by two signals (or components) in baseband and in quadrature i(t) and q(t), which are images of two binary streams representing a piece of information to be transmitted. In effect, whatever the type of digital modulation, the signal to be transmitted m(t) can be written:
m(t)=i(t)xc2x7cos(xcfx89t)xe2x88x92q(t)xc2x7sin(xcfx89t), where xcfx89 (=2xcfx80f) is the transmission frequency of the signal (also called the carrier frequency).
In the state of the technology, different types of radiofrequency transmitter are known, each based on a distinct architecture. The most widely known are the radiofrequency transmitter with frequency transposition, the radiofrequency transmitter with direct conversion and the radiofrequency transmitter with a phase locked loop. Their respective disadvantages will now be discussed.
The radiofrequency transmitter with frequency transposition which permits transposition to an intermediate frequency FI, requires the use of selective pass band filters, so as to reject the image frequency of the wanted signal to be transmitted. This first type of radiofrequency transmitter provides good performance, thanks to frequency transposition into the digital domain. However, the requirement to use high performance filters restricts its degree of integration onto silicon.
The radiofrequency transmitter with direct conversion has the most simple architecture and offers a high degree of integration. Its weak point is its high sensitivity to the performance of the elements that make it up. In particular, it is recommended that any leakage from the conversion oscillator via the mixer be avoided or that provision is made for perfect quadrature of the sine and cosine signals. These imperatives are often difficult to keep to.
The radiofrequency transmitter with a phase locked loop offers numerous advantages, such as the ability to do away with RF filters thanks to the pass band characteristic of the phase locked loop or PLL. The requirement to have the signals strictly in quadrature is also avoided. Nevertheless, these results are only possible if the voltage controlled oscillator or VCO included in the PLL provides high performance. This is not the case with integrated VCOs. Consequently, the PLL radiofrequency transmitter does not enable a high level of integration to be provided.
Therefore in a general way, these three types of known architecture offer a necessary compromise between integration, consumption and complexity. In other words, none of these three known solutions is entirely satisfactory.
A particular objective of the invention is to remedy these various disadvantages of the state of the technology.
More precisely, one of the objectives of this invention is to provide a radiofrequency transmitter providing good precision and offering a very high degree of integration on silicon.
Another objective of the invention is to provide such a radiofrequency transmitter that has very low sensitivity to the imperfections in the elements that make it up.
Another objective of the invention is to provide such a radiofrequency transmitter that enables one to avoid degradation of the wanted signal.
A complementary objective of the invention is to provide such a radiofrequency transmitter which is simple and is not any more complex than the known architectures.
Another objective of the invention is to provide such a radiofrequency transmitter that allows one to generate a resultant signal that has an image frequency that is sufficiently weak to be able to be suppressed by a filter with relaxed constraints (this filter thus being capable of being integrated).
In an embodiment variation, another objective of the invention is to provide a radiofrequency transmitter that does not give an image frequency, the image frequency at the output being completely attenuated, in an automatic fashion, by a self-calibrating system that compensates for imperfections both in gain and in phase.
These various objectives as well as others that will become apparent below, have been achieved according to the invention by a radiofrequency transmitter, of the type supplied with two signals in baseband and in quadrature, i(nT) and q(nT), which are images from two binary streams representing information to be transmitted, the radiofrequency transmitter comprising:
means of transposition into an intermediate frequency and of digital processing, that provide a first transposition into the digital domain, at an intermediate frequency xcfx890, for said base band signals, and generating, by combination, two signals at the intermediate frequency and in quadrature;
means of direct conversion, providing a second transposition into the analog domain, after multiplication by a frequency xcfx891, followed by a summation, of said two signals at the intermediate frequency and in quadrature, in a way that generates a resultant signal which is finally modulated around a frequency xcfx892, where xcfx892=xcfx890+xcfx891.
Therefore, this invention proposes an original architecture for a radiofrequency transmitter that combines the architectures with direct conversion and with frequency transposition, and which provides, in addition, means of digital processing, which provide preprocessing that permits attenuation at the output of the image frequency introduced by the means of transposition into an intermediate frequency. Hence, this new architecture combines the main advantage of the radiofrequency transmitter with direct conversion (no image frequency) with that of the radiofrequency transmitter with frequency transposition (no degradation of the wanted signal), while at the same time avoiding their disadvantages (sensitivity to imperfections, high performance filter).
In the description that follows, it will be shown that this invention operates perfectly if the two channels of the direct conversion means have the same gain and if the sines and cosines supplied by the oscillator included in the direct conversion means do not suffer from poor quadrature forming.
It will also be shown that, in the contrary case, a low power interference signal appears at the image frequency, but the wanted signal is not degraded in practice. Consequently, it is not essential to use, at the output, a filter attenuating the image frequency of the wanted signal. In any case, when the performance demanded from the transmission chain requires the use of such a filter, the latter can have relaxed constraints since the image frequency is very attenuated and can therefore be easily suppressed. In other words, the quality of the transmitted signal can be preserved without the need for high image filtering constraints. In certain cases, if the constraints are sufficiently relaxed, the image filter may possibly also be integrated with it.
It should be noted that the first frequency transposition and the signal processing are carried out in the digital domain, which enables one to benefit from the precision and the high degree of integration (on silicon for example).
It will also be noted that the radiofrequency transmitter according to the invention has a high degree of integration (for example, on silicon) and advantageously can even be entirely produced in the form of an integrated circuit. In effect, the means of direct conversion are known for their high degree of silicon integration. Furthermore, the level of integration of the means of transposition into an intermediate frequency can be relatively high since it is not necessary to use high performance filters. Finally, the digital processing means can be reduced to an assembly of elements currently used in integrated systems on silicon, and notably in transmitters with frequency transposition. This assembly of elements comprises, for example, a Numerically Controlled Oscillator or NCO and linear operators (multipliers and adders).
In addition, the extra complexity compared with a direct conversion architecture is negligible.
Finally, passing through a first intermediate frequency xcfx890 generated in the digital domain makes possible the attenuation of any possible leak from the conversion oscillator via the mixers.
In one advantageous embodiment of the invention, said radiofrequency transmitter additionally comprises means of digitally compensating for imperfections in gain and in phase in said means of direct conversion.
Hence, by ensuring that at the output of the radiofrequency transmitter, the signal at the image frequency is completely attenuated, the performance of the radiofrequency transmitter according to the invention is optimized and the resulting transmitted signal has characteristics close to the ideal case. Thanks to this self-calibrating technique of image annulment, the errors introduced by the analog part (that is to say the means of direct conversion) sensitive to the imperfections, are compensated for in the digital domain.
It is important to note that in this particular embodiment, no image frequency filter whatsoever is required. This new architecture for a radiofrequency transmitter therefore operates independently of the chosen carrier frequency and is therefore particularly suitable for multi-standard systems of radiocommunications. Among the standards possible, one may mention only by way of examples, the Global System for Mobile communications or GSM, the Digital Cellular System 1800 MHz or DCS 1800, the Personal Communication System or PCS 1900, Digital European Cordless Telecommunications or DECT or the Universal Mobile Telecommunication System or UMTS etc.
Preferably, said analog/digital conversion means have a working frequency substantially identical to the working frequency of the digital/analog conversion means in said means of direct conversion.
In a preferred way, said means of digital compensation are included in said integrated circuit. Hence, the radiofrequency transmitter according to this invention can be entirely integrated, for example on silicon.